Due to the increasing energy consumption and shortage of petroleum resources around the world as well as the soaring oil price, the demand for bio-diesel fuel, a kind of renewable energy source, is increasing day by day. The production is gaining momentum and techniques employed in production mainly involve refinement of the soybean oil, colza oil to the extent that the concentration of free acid achieves less than 0.1%. It is also achieved through synthesizing with methanol solution of alkaline catalyzer by means of ester-exchange reaction.
For raw materials with a high concentration of free fatty acid, the means of alkaline ester exchange reaction isn't applicable. Instead, free fatty acid should be reduced by using acid catalyst ester before the alkaline ester exchange reactions are applied.
Raw materials with high free fatty acid content are also esterified and undergo ester exchange reaction by means of getting mixed with acidic catalysts. Some also adopt the methods of hydrolyzing oil first into fatty acid, then esterifying it with acidic catalysts or by applying pressure. Some others are supercritically esterified under the high temperature and high pressure.
All in all, all the methods have some shortcomings, i.e., the method of alkaline ester exchange reaction is highly costly as it requires refined soybean oil and colza oil as raw materials. Acidic catalyst ester exchange reaction method, which needs enamel facilities due to its strong corrosive property, is not appropriate for production on a large scale and the large amount of sulfuric acid consumption causes severe environmental contamination. High-temperature and high-pressure or supercritical esterification entails advanced alloy materials due to its strong corrosive properties. Hence, a large amount of investment and fairly high energy consumption are required due to immediate cool-down after high-temperature reaction.